This section provides background information to facilitate a better understanding of the various aspects of the disclosure. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art.
In the technical field of hydraulic fracturing, one major challenge impeding an understanding of the detailed mechanics of fracturing, proppant flow, and closure behavior is an inability to actually view the process. Rock, most often being at least opaque, obscures any view of what is occurring therein, and typically, the subterranean formation operation occurs 5,000 to 20,000 feet away from the surface. Further, in some cases, important cause and effect events take place tens to hundreds of feet away from the wellbore deeper into the subterranean formation region into which the fracture extends.
Many approaches to better understand the detailed effects of fracturing processes have been attempted over the years. One approach includes a methodology referred to as ‘mine back’. In a mine back operation, colored sand is placed in a fracture formed near an existing mine in a subterranean formation. Once the hydraulic fluid pressure is decreased and the fracture has closed upon the sand, the mine is further extended toward the fracture in layers. The sand deposits are recorded with photos as they are revealed. Such an approach is known to be expensive and time consuming, as well as limited to very special situations and formations.
Some other approaches have included use of microseismic and tilt meter measurements to ascertain some degree of understanding of the extent and dimensions of downhole fractures in real wells. Also, radioactive sand has been used to comprehend near well bore sand distributions, to some limited degree. In another approach, chemical tracers pumped in with the fracturing fluid were used provide some indication of fracture length due to the time that elapsed for the tracer to flow back to flow back.
On a large laboratory scale, blocks of rock and ice that have been fractured at scales of up to 1 meter on a side have been used. The fractures are made in the blocks at significant hydrostatic and confining pressures in order to ascertain the mechanisms that may be involved in fracturing. Also, slot flow cells have been used to visualize simulated behavior of fracturing fluids as they are pumped into the cells. These cells typically consist of two parallel transparent plates of acrylic or polycarbonate with a spacer between to define a gap. Often, structural members constrain the edges and in some cases, may run across the face of the cell. Also, these cells all suffer from significant wall deflections, as well as have substantially smooth walls. In some cases, cells used about 1″ thick acrylic plates for the viewing ports and also for the flow surface. This meant that a damaged flow surface necessitated replacing the pressure containment at a significant cost. Also, it was not economically feasible to have a very stiff cell, and the deflection of 1″ acrylic over a 4′ span was easily visible with only hydrostatic pressure due to water.
It remains an ongoing need to provide improved methods and apparatus for understanding of the detailed mechanics of formation fracturing, proppant flow, and fracture closure dynamics, such need met, at least in part, by embodiments disclosed in the following disclosure.